220 



CURTIS: MODERN THEORIES OF SPIRAL NEBULAE 



No spiral has as yet been found actually within the structiire 

 of the Milky Way. We have doubled and trebled our exposures 

 in regions near the galactic plane in the hope of finding fainter 

 spirals in such areas, but thus far without results. The out- 

 standing featiu"e of the space distribution of the spirals is, then, 

 that they are found in greatest profusion where the stars are 

 fewest, and do not occur where the stars are most numerous. 

 This distribution may be illustrated graphically as follows: 



THE FACTORS OF SPACE DISTRIBUTION 



400,000 =•= Spiral Nebulae 



Our own stellar universe 



is shaped like a thin lens, and is perhaps 



3,000 by 30,000 Hght-years in extent. In this 



space occur nearly all the stars, nearly all the new stars, nearly 



all the variable stars, most of the diffuse and 



planetary nebulae, etc., but no spiral 



nebulae. 



300,000 =*= Spiral Nebulae, 



4. The spectrum of the spirals is practically the same as that 

 given by a star cluster, showing a continuous spectrum broken 

 by absorption lines. A few spirals show bright-line spectra 

 in addition. 



